So I recently added two entries for the super-heavy tank Ratte, and I was wondering... Would this be a "proto/concept" instead of the "military armored vehicle? I know Wikipedia calls it a prototype precisely since Krupp never was able to get this Wunderwaffe off the ground (hence why both entries are from Animes), but the "proto" category seems to not apply for military vehicles AFAIA.

You have to check also in what group is each vehicle type. "Proto/concept" is in the "Cars" group.
That one is mostly for concept-cars (those we see in auto salons) more than prototypes (early builds for tests).
So for tanks even if they are prototypes they can go in the military armored category, in the "Others" group.

Understood. Thank you for your input.